As frequency bands for mobile communication of portable phones, GSM (880 to 960 MHz) and DCS (1710 to 1880 MHz) are used in Europe, AMPS (824 to 894 MHz) and PCS (1850 to 1990 MHz) are used in the United States, and PDC 800 (810 to 960 MHz) and PDC 1500 (1429 to 1501 MHz) are used in Japan. Then, as a built-in antenna of a portable phone, an antenna capable of transmitting and receiving two frequency bands respectively corresponding to areas where the equipment is used is generally used.
An example of a structure of this conventional dual band antenna for mobile communication will be described with reference to FIG. 29. FIG. 29 is an outer appearance perspective view of the example of the structure of the conventional dual band antenna for the mobile communication. In FIG. 29, a grounding plate 12 is disposed on substantially the whole surface of a circuit board 10. A carrier 14 made of a dielectric is disposed on the circuit board 10, and a metal plate 16 of a good conductor functioning as an antenna element is disposed on the upper surface of this carrier 14. A suitable slit 16a is provided in this metal plate 16 to make a suitable form, a suitable position of the metal plate 16 and the grounding plate 12 are electrically connected to each other by an earthing terminal 18 made of a spring connector or the like, another suitable position of the metal plate 16 and a terminal 10a of the circuit board 10 are electrically connected to each other by a feed terminal 20 made of a spring connector or the like, and a first and a second antenna elements functioning as inverted-F antennas respectively resonant at a first frequency band and a second frequency band are formed of the metal plate 16 provided with the slit and having the suitable shape. The first frequency band is one of the GSM, AMPS and PDC 800, and the second frequency band is one of the DCS, PCS and PDC 1500.
Here, in case the dual band antenna is incorporated in a chasis of a portable phone, a width W is restricted to about 40 mm. On the other hand, the wavelength is shortened according to the dielectric constant of the carrier 14, and as the dielectric constant of the carrier 14 becomes high, the size of the antenna becomes small, however, the gain becomes small by that. Besides, as the dielectric constant becomes low, the size of the antenna becomes large and the gain becomes large, however, it cannot be accommodated in a desired space. Then, when it is incorporated in the portable phone, it is desirable that the size of the antenna is made as large as possible within a range where it can be accommodated, and the gain becomes large in some degree. For that purpose, it is desirable that the carrier 14 is formed with a desired dielectric constant. However, the carrier 14 cannot be always formed of a suitable material from the viewpoint of manufacture or cost. Then, the carrier 14 is provided with a hollow part 22 and is formed to have a substantially C-shaped form with a top plate part 14a and both side parts 14b and 14b, and a desired dielectric constant in total is obtained by a dielectric constant of a material of the carrier 14 and a dielectric constant of the air in the hollow part 22.
Incidentally, although the metal plate 16 may be formed by sheet metal processing, it is a matter of course that the metal plate may be formed of a thin film of a good conductor member suitably provided on the upper surface of the carrier 14 by resin plating, hot stamp, evaporation, etching or the like.
In recent years, with comings and goings of many people between the United States and Europe, it is desired that one portable phone can be used in both the United States and Europe. Then, it is desired to realize a broad-band antenna which can transmit and receive a first frequency band intended for the GSM of Europe or the AMPS of the United States or having both the GSM and the AMPS in the band, a second frequency band intended for the DCS of Europe, and a third frequency band intended for the PCS of the United States. Besides, with the rapid development of a technique for mobile communication, IMT-2000 (1920 to 2170 MHz) higher than the conventional frequency band and used in common all over the world is proposed. Then, it is also desired to realize a broad-band antenna capable of transmitting and receiving a fourth frequency band intended for the IMT-2000.
However, if three or four antenna elements capable of being respectively resonant at the foregoing three or four frequency bands are provided on the surface of the carrier 14, the total size becomes large, and they can not be incorporated in the portable phone chassis. Besides, when they are daringly formed to have such sizes that they can be incorporated, the respective antenna elements excessively come close to each other, interference occurs among them, and a desired antenna characteristic can not be obtained.
Accordingly, the present invention has an object to provide a broad-band antenna for mobile communication which can obtain a desired antenna characteristic in plural frequency bands.